If you’re among those who believe that Arrow‘s 100th episode should have been marked independent of this week’s “Heroes v Aliens” crossover event, here’s a little secret: Next week is also the 100th episode. In a way.

Because whereas Arrow‘s crossover episode — titled “Invasion!” and airing Wednesday at 8/7c on The CW — offers up an early Valentine to fans by depositing Oliver in a reality where Moira, Laurel and other loved ones never perished, “What We Leave Behind” (airing Dec. 5) has a special something of its own going on.

“There was a lot of temptation to do the crossover at a different time or, like you [suggested], ‘cheat the math,'” Arrow co-showrunner Marc Guggenheim shared with TVLine in an in-depth interview, “but I actually took an idea that I had for the 100th episode and moved it to the 101st, and that is ‘flashbacks to Season 1,'” as a new clue surfaces linking the villain Prometheus to Oliver’s past. “That was something that I wanted to do for the 100th episode, but there just wasn’t any room to tell a flashback story.”

Instead, using that nostalgic narrative device back-to-back with this week’s guest star-studded alternate reality “worked out really well,” he notes.

Having been with Arrow from go, Guggenheim needs neither alien mind control nor flashbacks to appreciate all that the series has brought to the table, up to and including spawning not one but three other superhero shows, all woven into this week’s #DCWeek event. In the photo gallery above right, the EP shares favorite memories and musings from the past 100 episodes — Which casting was a “happy accident”? What stunt still wows him? Did “Olicity” happen too soon? — as well as offers some behind-the-scenes secrets.